In these times of shortages, copper may be added to the list of raw materials in short supply and increasing demand. This situation raises prices and creates an enthusiastic interest in consumers for substitute materials. One of the most viable substitutes for copper has long been aluminum. This material is less expensive than copper and its electrical properties are only slightly less attractive. As a result, there has been a considerable effort expended to substitute aluminum in those applications where the losses created by the less attractive electrical properties would be tolerable.
However, aluminum has not been successfully substituted for copper in commutator bars designed for low voltage applications. There are two reasons for this failure. First, aluminum is not physically as strong as copper, and because of its relative weakness, the conventional process of forming the commutator and fastening the bars to the spool cannot be used. In this process the commutator spool is molded around projections, called retention hooks, which extend inwardly from each bar into the spool. The process requires relatively high molding pressures and aluminum does not have the strength to withstand the forces necessary to form the spool, and the projections are crushed. Secondly, aluminum, when exposed to the atmosphere, forms an oxide (i.e., alumina) layer on its surface. The alumina-aluminum junction is an electrical valve or diode; that is, the electical resistance in one direction across the junction is less than it is in the opposite. In general, the net resistance of this oxide is relatively high, but in high voltage applications it presents few problems. However, in applications such as the automotive 6 or 12 volt systems the increased resistance significantly reduces the performance of any motor equipped with aluminum commutator bars.
To date these problems have effectively precluded the use of aluminum alloy commutator bars in low voltage applications.